1. Field of the Invention
The present invention relates to an electroacoustic transducer used for an acoustic device such as a speaker, and an electroacoustic transduction system.
2. Description of the Related Art
In response to thinning of displays such as liquid crystal displays and organic electroluminescence (EL) displays, speakers used in such thin displays are also required to be lighter and thinner. As such a lightweight and thin speaker, it is considered to adopt a sheet-like piezoelectric film having a property that stretches and contracts in response to an applied voltage.
For example, in JP2008-294493A, it is described that a piezoelectric film obtained by performing polarization processing with respect to a monoaxially stretched film of polyvinylidene fluoride (PVDF) at a high voltage is used.
In order to adopt the piezoelectric film as a speaker, it is necessary that a stretching and contracting movement along a film surface is converted into a vibration of the film surface. This conversion from the stretching and contracting movement into the vibration is attained by holding the periphery of the piezoelectric film in a bent state, and thus the piezoelectric film is able to function as a speaker.
However, since the piezoelectric film formed of monoaxially stretched PVDF has an in-plane anisotropy in the piezoelectric properties thereof, when the entire peripheral portion thereof is fixed during bending, the vibration mode is disturbed, and sufficient sound volume and acoustic quality are not able to be obtained. Furthermore, since PVDF has a smaller loss tangent than a general vibration plate for a speaker such as cone paper, a strong resonance is easily generated, and near the resonance frequency determined by a radius of curvature when the film is held while being bent, a large number of peaks and dips occur on the sound pressure and frequency properties. Thus, in a lightweight and thin speaker using the piezoelectric film formed of PVDF, it is difficult to reproduce a sound with high acoustic quality.
Here, the present applicants suggested, as a lightweight and thin speaker capable of reproducing a sound with high acoustic quality, an electroacoustic transduction film including a polymer composite piezoelectric body in which piezoelectric body particles are dispersed in a viscoelastic matrix formed of a polymer material having viscoelasticity at a normal temperature, thin film electrodes formed on both surfaces of the polymer composite piezoelectric body, and protective layers formed on the surfaces of the thin film electrodes, which is disclosed in JP2014-14063A.